Integrated circuits are often fabricated with one or more devices, which may include diodes, capacitors, and different varieties of transistors. These devices often have microscopic features that can only be manufactured with critical processing steps that require careful alignment of equipment used to build the devices. These microscopic features contain critical dimensions that will often define the performance of the device and its surrounding circuitry.
Additionally, the functionality of these devices is defined by creating precisely controlled regions of dopants within the various layers of a semiconductor wafer. These dopants, however, are susceptible to diffusion at elevated temperatures.
Semiconductor fabrication continues to advance, requiring finer dimensional tolerances and control. Modern integrated circuit design has advanced to the point where line width may be 0.25 microns or less, with junction depths on the order of 1500–2000 Angstroms. Thus, thermal effect to a semiconductor wafer must be reduced to limit the lateral diffusion of the dopants, and the associated broadening of line dimension. Thermal effect to a semiconductor wafer must also be limited to reduce forward diffusion of the dopants so junction depth does not shift.
An additional potential adverse effect of thermal treatment is chemical change of the materials utilized in the fabrication of a semiconductor wafer. As an example, refractory metal-silicide films may be formed during the fabrication of very large scale integration (VLSI) circuits as a gate or interconnect film. These refractory metal-silicides can be reduced to their elemental constituents under elevated thermal conditions, thus destroying the functionality of the device.
In an effort to reduce the magnitude of thermal requirements, and thus lessen the likelihood of adverse effects, differing processes have been developed. One such process is Rapid Thermal Processing (RTP). RTP is a short-duration, high-temperature, radiant-heating process. RTP may be found in a multitude of semiconductor fabrication processes in a variety of forms, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN).
RTP seeks to minimize the negative effects of necessary thermal treatments, and thus reduce the thermal budget of a semiconductor wafer, by subjecting the semiconductor wafer to high temperatures (typically 420–1150° C.) only long enough to achieve the desired process effect. Systems are commercially available to perform RTP and generally utilize large-area incoherent energy sources such as radiant heat lamps operating in the wavelengths of 0.5 to 3 μm. RTP systems typically thermally isolate the semiconductor wafer being processed such that radiant heating is the dominant mode of transfer, seeking to minimize heat transfer by conduction at the wafer surface or convection around the wafer surface.
RTP was started as a research technique some 25 years ago using pulsed laser beams. As the semiconductor industry continues its trend into submicron devices, RTP is becoming a core technology step in the development and mass production of ultra-large system integration (ULSI) devices. Since their introduction more than a decade ago, RTP processors employing incoherent lamps are now the mainstay.
Despite these prior improvements to thermal processing, difficulties still exist. To achieve desired semiconductor device characteristics, temperatures and dopant implant depths must be adjusted within small process windows to achieve the desired process effect while compensating for the inevitable diffusion. As device sizes are reduced, control of implant diffusion becomes more critical. Reducing thermal effect to the semiconductor substrate relaxes and widens the available process window for control of implant diffusion.
Furthermore, in many semiconductor fabrication steps requiring heat input, the desired process effect relates only to the surface of the semiconductor wafer. In these cases, there is no need or desire to provide heat input to the substrate. It should be noted that all references to the substrate within this disclosure shall include all layers underlying the surface.
One such use of RTP is for a process known as contact reflow following anisotropic etching. Etching in semiconductor fabrication involves the removal of material from the wafer surface. Anisotropic etching is typically used to describe etching occurring only in the direction perpendicular to the wafer surface. This vertical etching results in sharp edges on contact holes, making them difficult to fill. Where flowable glass, such as borophosphosilicate glass (BPSG), is used for isolation, passivation or surface planarization, these sharp edges may be rounded through the process of contact reflow.
Contact reflow involves the heating or annealing of the glass to cause reflow and, therefore, rounding of surface features. With rounded surface features, coverage of contact holes by subsequent metal film layers is improved. Because the process seeks only to round the surface features of the glass surface layer, heat input to the underlying layers is neither required nor desired.
In light of the foregoing, it may be desirable to selectively increase the processing temperature of the surface layer of a semiconductor wafer while reducing the thermal effects to the substrate that would normally occur in pre-existing rapid thermal processing.